1. Field of the Invention
The present invention relates to a method of providing a coating or film on the surfaces of materials exposed to molten metals. In particular, the invention relates to a method of coating a metal part to make it resistant to soldering or “liquid metal attack” when exposed to molten metals, by applying to one or more surfaces of the metal part a composition containing a liquid metal carboxylate, followed by heat treatment of the coated metal part.
2. Description of Related Art
The metal casting industry provides more than 90% of all manufactured goods and capital equipment for manufacture of other goods in the United States. It is a significant component in the manufacture of automobiles, trucks, agricultural and construction equipment, railroads, pipes, fittings, valves, and engines. In the U.S. alone, there are approximately 3000 operating foundries, employing around 250,000 people.
A significant problem in the metal casting industry, and one which decreases energy efficiency in an already energy intensive industry, is termed “liquid metal attack.” Liquid metal attack occurs when the molten metal being cast adheres or “solders” to the mold or die. Liquid metal attack can occur where solid metal parts, such as molds, dies, ladles, sheaths, pins, tooling, and other parts, come into contact with molten metal. This can occur during die casting, smelting, or other stages of metal processing where molten metal contacts a solid metal material. The molten metal causes wear and erosion of the solid metal parts, increasing both capital costs and costs associated with increased labor and downtime.
In addition, efficiency of die casting processes are reduced by liquid metal attack, and the scrap rate of die casting processes is increased. For example, the aluminum die casting industry melts about 2 lbs of aluminum for every pound of die cast aluminum shipped. An increase in yield of only 20% would save approximately 2.7*1012 BTU/yr.
A method of preventing liquid metal attack on solid metal tools used to process molten metals would provide an enormous cost and energy savings to a number of industries, not limited to the die casting industry, but extending to those industries that rely on metal casting to provide critical parts. Before this invention, no coating solution had been found that sufficiently withstands the corrosive nature of molten aluminum. Extending the useful lifetime of dies would significantly reduce time between costly die replacements.
Moreover, to the inventor's knowledge, coating techniques have not been described that provide the ability to do an “in field” coating of metal parts that is resistant to liquid metal attack. To the contrary, coatings used to limit wear, erosion, or corrosion for die-casting dies are typically applied in a super controlled atmosphere, e.g., by physical vapor deposition (PVD) or chemical vapor deposition (CVD), which can require elaborate safety equipment. This presents a disadvantage because it becomes economically infeasible to apply a CVD or PVD “in field.” In addition, such coatings, such as CrN, CrC, BC, VC, CrN2, and ion nitriding, often fail by spalling due to mismatched thermal coefficient of expansion (TCE) with the solid metal substrate. This is in part because of the relatively thick coatings (5 to 15 μm) that can be applied with these techniques.